This invention relates to a thread cutting device for a double lockstitch sewing machine provided with a hook rotatably driven about an axis parallel to the needle. In particular, the invention relates to a thread cutting device provided with a knife arm tiltably arranged about an axis parallel to the hook axis and provided with a thread catcher including a cutting edge, at which the thread catcher is formed with a leg directed towards the thread catching direction so as to catch particular threads during the thread cutting operation.
A thread cutting device of this type is known from U.S. Pat. No. 3,624,735 in which the thread catching arm is formed with a cutout so as to catch the bobbin thread leaving the bobbin case at an outlet opening. In operation, the thread catcher of the thread cutting device is tilted in the rotating direction of the hook point when the needle thread loop has been pulled out completely around the bobbin case and especially at an instant when already the needle thread point leading towards the workpiece to be sewn is positioned in a lower area and when the portion of the needle thread leading towards the needle is positioned in the upper region. In order to catch the portion of the needle thread leading towards the workpiece the upper leg of the thread catcher having a leading point limits the corresponding cutout towards the upper direction. Furthermore, this leg has an upwardly overbent profile so that the upper positioned part of the needle thread leading towards the needle will be deflected upwardly thus not caught by the thread catcher. The catching of the threads is carried out at an instant when the needle thread surrounding the bobbin case starts to fall off. In this instant the threads to be caught by the thread catcher are relatively loose. At this known thread cutting device a specially profiled finger preventing a rotation of the bobbin case together with a hook body must be arranged at the bobbin case at the area of the stitch hole of the throat plate as to render possible the movements of the thread catcher.